Flame ionization detector



E. M. TAFT Aug. 27, 1968 FLAME IONI ZAT ION DETECTOR Filed March 22, 1965 F/GZ FIG!

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A TTORNEYS United States Patent 3,399,039 FLAME IONIZATION DETECTOR Earl M. Taft, Lafayette, Caliil, assignor, by mesne assignments, to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 22, 1965, Ser. No. 441,826 4 Claims. (Cl. 23254) This invention relates to a hydrogen flame ionization type detector for use in detecting materials in gas chromatography.

In gas chromatography packed columns are employed to separate materials in gaseous form at phase intervals. Such materials are generally present in very minute quantity and are detected by detectors having extreme sensitivities. One type of detector which has been successfully employed is classified as a hydrogen flame ionization type detector in which gas is flowed into a hydrogen flame and one electrode is mounted adjacent the flame and another electrode is mounted more remote from the flame. The two electrodes form substantially cathodic and anodic electrodes in which the current flow at the anode is determined by the amount of material ionized within the gap by virtue of the hydrogen flame. It had been the previous practice to form the cathode in a ring close to but spaced from the candle from which the hydro-gen flame is emanated. In such detectors it was found that sensitivity decreased when the temperature ranges were raised to the higher levels. The precise [reason for this phenomenon is unknown, but it is surmised to have occurred because of electrical leakege through the quartz candle. This may occur because of a change in conductivity of the quartz at higher temperature or by virtue of contaminance deposited on the quartz. In any event the practical eliect of such candles was an actual loss of sensitivity when operating beyond certain temperature ranges. In some detectors the candle is formed of a conductive material so that the candle functions as the cathodic electrode. With a metal candle the maximum operating temperature is also limited. This apparently occurs because as the temperature increases towards the upper level the heat afforded by the flame causes secondary emission which in turn aflects the accuracy and sensitivity of the detector. Both the nonconductive and conductive candles therefore have severe limitations as to the maximum temperature range at which it can be effectively operated.

The object of the present invention is to provide a flame ionization detector in which the cathodic electrode is mounted to contact the candle at a position below the flame sufliciently to isolate the electrode from a substantial portion of the flames heat and yet maintain a conductive discharge path through the electrode and into the readout system. By virtue of this expedient it has been found that the temperature range at which the detector can operate at adequate sensitivity can be extended considerably.

Another object of the present invention is to provide a hydrogen flame ionization detector housing a flame emitted from a candle formed of non-conductive material and in which an electrode of the type that can be heated sufliciently to cause ignition of the hydrogen flame is placed in physical contact with the stem slightly below the point of flame emission.

A feature and advantage of this device lies in the fact that metal forming the electrode is sufficiently remote from the flame to minimize secondary emission and at the same time the candle is connected directly into the readout system of the detector at a point which tends to minimize the eflect of higher conductivity resulting from increased temperatures employed within the detecting device.

These and other objects and advantages of the invention will be apparent upon reading the detailed description in conjunction with the drawings in which:

FIG. 1 is a schematic view showing a typical hydrogen flame ionization detector incorporating the present invention;

FIG. 2 is an exploded view of the stem of FIG. 1 showing the contact of the electrode to the stem;

FIG. 3 shows an alternative embodiment of the invention in which the electrode is formed in a ring configuration surrounding the tip of the stem.

In the present invention there is provided a flame ionization detector A which generally comprises a tube 15 having at the bottom an entry inlet or port 16 for the introduction of test gas. At the top of tube 15 there is pro vided a cap 18 opening into a funnel 19 which in turn opens to an escape port 21. Housing 15 is formed of a conductive material to allow the housing to function as an anode or positive electrode. A burning pedestal B is arranged in coaxial alignment with housing 15 in which an upper stem or candle 25 is arranged to project upwardly into the central portion of the housing and in axial alignment therewith. The pedestal is formed of non-conductive material which is stable at high temperatures such as quartz and is formed with a canal 30 through which hydrogen gas is flowed via an inlet tube 31. The test gas from port 16 is mixed in canal 30 with the hydrogen. Air to support combustion of the hydrogen is flowed into the base via a hose 33 and an outlet jet 34 mounted to discharge the air inside the burning chamber.

The outside contour of tip 35 of stem 25 is many times formed in a conical shape to form the burning exit 36 for flame 37. The body of the burner is supported by a conventional base structure 38 which may either be floating or ground potential. An electrode ignitor 40 is mounted in cont-act with the conical portion 35 of stem 25 just slightly below the exit port 36. The ignitor is formed of a filamentary wire which is connected by two leads 42 and 43 to a source of power 45 which when actuated through the closure of switch 46 will cause the filamentary wire of the electrode 40 to heat up to reach the kindling point of the hydrogen and thereby ignite the hydrogen to form flame 37. One of the terminal leads to the ignitor or cathode 40 is connected to the negative terminal "of power source 48. The positive terminal of the power source is connected through a load resistance 49 to housing 15. The voltage drop across load 49 is rendered on a readout mechanism 50 to give an indication of the current flow between cathode 40 and anode 15. In operation, the hydrogen and test gas are flowed into gas inlet 31 and 16, respectively, and after mixing are emitted from the gas outlet port 36. At this time switch 46 is closed to cause ignitor 40 to light the hydrogen. Gas flowed into input entry 16 into the test area defined by the anodic housing 15 is heated by the hydrogen flame to effect ionization of the gas molecules. The gas input is obtained from typical gas chromatography columns in which minute quantities of gas are emitted within a carrier gas at phase intervals. The relative phase interval of emission is a qualitative indication. In this device the current flow at the anode 15 is directly related to the amount of ionized gas existing between the two electrodes.

The device of the present invention employs electrode 40 in direct contact with quartz stem 25 wherein in prior art devices the ignitor cathode has been held in spaced relation to the stern. It has been found that in the prior art devices, when the temperature exceeded 250 C., there was a sharp drop-off of sensitivity. However, in the present invention with the ignitor cathode disposed in contact with stem 25, the temperature can be raised as much as 600 C. without appreciable fall-off of sensitivity. The point of connection of the cathode to the stem is at a position which locates cathode 40 sufliciently outside of the direct heat of the flame so that it will prevent thermal caused secondary emission from the cathode and is preferably as close to the tip as possible to relate the cathode to the flame. It is believed that the losses due to higher temperatures are caused by current leakage along the quartz stem either due to the effect of contamination on the stem wall or to changes of quartz resistivity at higher temperatures. In either event the connection of the electrode to the quartz adjacent the tip minimizes the aforesaid phenomena and allows the sensitivity to be maintained at the high temperature ranges.

While electrode 40 has been described as having a point contact with tip 35 it is apparent that a ring type cathode, as indicated at 55 in FIG. 3, can be employed wherein the cathode is formed of a resistance wire which forms a ring in contact about the upper end of the tube.

While several embodiments of the invention have been shown and described it will be apparent that other adaptations and modifications can be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. A detector for use in gas chromatography comprising an electrically conductive hollow housing, means to emit gas up into said housing, a flame burning candle formed of non-conductive material mounted within said housing, means supplying positive potential to said housing, electrode means supplying negative potential, said electrode means being in contact with a portion of the tip of said candle and being positioned outside of the direct flame area of the flame.

2. A detector for use in gas chromatography comprising an electrically conductive hollow housing, means to emit gas up into said housing, a flame burning candle formed of non-conductive material mounted within said housing, means supplying positive potential to said housing, an electrode positioned in said housing below the lower point of the flame, said electrode mounted in direct electrical contact with said candle, and means to supply negative potential to said electrode.

3. A detector in accordance with claim 2 and wherein said electrode comprises a filament and means to apply power through said filament to cause said filament to heat snfficiently to ignite said candle, said electrode being sufliciently near the tip of said candle to cause ignition of said flame.

4. In a detector for use in gas chromatography of the type having an electrically conductive hollow housing, means to emit gas up into said housing, a flame burning candle formed of non-conductive material mounted with in said housing, means supplying positive potential to said housing, an electrode mounted within said housing adjacent the flame, and means supplying said electrode with negative potential, the improvement comprising positioning said electrode in contact with said candle slightly below the flame area.

References Cited UNITED STATES PATENTS 2,991,158 7/1961 Harley 23-232 3,027,241 3/1962 Andreatch et al. 23-232 3,086,848 4/1963 Reinecke 23-232 3,175,886 3/1965 Krzeminski 23232 X MORRIS O. WOLK, Primary Examiner.

R. M. REESE, Assistant Examiner. 

1. A DETECTOR FOR USE IN GAS CHROMATOGRAPHY COMPRISING AN ELECTRICALLY CONDUCTIVE HOLLOW HOUSING, MEANS TO EMIT GAS UP INTO SAID HOUSING, A FLAME BURNING CANDLE FORMED OF NON-CONDUCTIVE MATERIAL MOUNTED WITHIN SAID HOUSING, MEANS SUPPLYING POSITIVE POTENTIAL TO SAID HOUSING, ELECTRODE MEANS SUPPLYING NEGATIVE POTENTIAL, SAID ELECTRODE MEANS BEING IN CONTACT WITH A PORTION OF THE TIP OF SAID CANDLE AND BEING POSITIONED OUTSIDE OF THE DIRECT FLAME AREA OF THE FLAME. 